The invention relates to a method for producing rolls or coils from films of insulating material coated under vacuum with electrically conductive material. The film is guided at a specified angle of contact over a roller rotating with the film and coating the film during its contact with the roller by vaporizing the conductive material by direct electron beam bombardment and condensing the vapor on the film surface. After this procedure the coated film is coiled up on a take-up drum.
The method referred to above is known from the German published patent application No. 3,420,245. The film of insulating material is made from a synthetic thermoplastic material; the electrically conductive material is a metal alloy of a magnetic material, such as a cobalt-nickel alloy. Magnetic tapes for audio and video recordings, for example, are produced by this known method.
This method of producing coated films is suitable not only for use with such films of insulating material in conjunction with electrically conductive magnetic materials, but also for the production of so-called capacitor films, which comprise a substrate of thermoplastic film and an aluminum layer. Neither the film material nor the electrically conductive coating sets limits to the range of applications of the inventive method.
With such a method, the handling of the film on its path from the coating source, that is, the electron beam vaporizer--to the take-up drum proves to be especially problematical. First of all, the film must be pulled from the roller, which frequently is also referred to as a cooling roller or a vapor-deposition roller, with the expenditure of not inconsiderable forces, because negative charge carriers, which bring about electrostatic charges, are implanted in the film by the vapor-deposition process, so that the film adheres to the roller due to electrostatic forces. Admittedly, this may be considered a positive feature insofar as the electrostatic "suction" promotes the heat dissipation to the roller and thus the cooling of the film. On the other hand, however, it leads to considerable coiling problems, because the tensile stress produced by the effect described leads to extremely undesirable fold formation in the film. The tendency to form folds increases wih the following parameters:
increasing voltage of the electron beam, PA1 increasing film width, PA1 increasing number of deflector rolls, PA1 decreasing film thickness, and PA1 increasing temperature difference (quenching effect).
An attempt has already been made to produce a stretching effect in the transverse direction of the film with curved deflector rolls (the so-called "banana rolls") and, by so doing, to counteract the formation of folds. This measure also has only a very limited effect.
The absence of the electrically conductive coating at the two side edges of the film has a particularly disavantageous effect. These two coating-free edge strips--as well as the uncoated center strip--are required for capacitor films; they are, however, also especially desirable so as not to coat the length of roller which necessarily projects beyond the film, since this would lead to an edge bead that becomes increasingly thicker. The uncoated edge strips have a particularly disadvantageous effect during the build-up of the coil, and moreover, as the coil diameter increases, the build-up of a regular edge bead with a partial doubling over of the film may be observed. It has hitherto not been possible to eliminate this effect, so that only a coil with a limited diameter could be produced.
Electrons may be regarded as the cause of the formation of folds and of the so-called edge beads on the coil. These electrons are reflected from the surface of the material to be vaporized from the so-called bath mirror, and have energies up to the energy of the electron beam. They are implanted in the film and can charge this film up to a voltage of 30 kV. This charge causes the film to "adhere" not only to the vapor-deposition roller, but also to the subsequent deflector rolls. On the finished coil, this charge also leads to an attraction of the adjacent coil layers in the region of the conductive coating. It may be assumed that the cause of this attraction is a polarization within the coated film. This attraction prevents the gliding process of the individual film layers over one another, which is necessary for a fold-free coil. On the other hand, when uncoated edge strips are present, repulsion of the individual film layers takes place in the region of these strips and finally leads to the above-described build-up of an edge bead in the coil.